With the rapid development of Internet and the appearance of various multimedia services, the needs for the mobile communication is not limited to the telephone and message services, nowadays, application service is introduced to the multimedia services, and the feature of the application service is that a plurality of users could simultaneously receive the same data, such as video on demand, TV broadcasting, video conference, online education, and interactive games.
In order to use the mobile network resource effectively, multimedia broadcast multicast service (MBMS) technique is proposed, and the MBMS is a point to multipoint service in which a data source sends data to a plurality of users. With this service, the network resource including the mobile core network and access network resource, especially the air interface resource, can be shared. Moreover, the MBMS in 3GPP can implement not only the multicast and broadcast of low speed pure text messages, but also the multicast and broadcast of high speed multimedia service.
The MBMS single frequency network (MBSFN) is a technique which can improve the utilization efficiency of the MBMS service frequencies. In the MBSFN, the adjacent cells send the same radio signal synchronously, and the user equipment (UE) could consider the signals from different base stations as a multi-path signal. A plurality of cells which send the MBMS service radio signals synchronously consist in the MBSFN-synchronized area, and the cells in the MBSFN-synchronized area send the same MBMS service signal synchronously, and these cells are called MBSFN cells.
Since the MBMS service is a service facing to all networks, the same MBMS service can be established in different lower layer network element nodes, specifically, an upper layer network element sends data stream to lower layer network elements, wherein, the data stream comprises a plurality of data bursts, and each data burst comprises a plurality of data packets. At present, the following method is applied for a plurality of network elements to implement the MBMS services synchronization among the cells. FIG. 1 shows the block diagram of the logic structure of the upper layer and the lower layer network elements. From FIG. 1, it can be seen that one upper layer network element connects with a plurality of lower layer network elements to interact signaling. FIG. 2 is a flow chart of the method for implementing the MBMS synchronization in the prior art, and as shown in FIG. 2, the method comprises the following procedures:
S202, an upper layer network element sends MBMS service data packets to every lower layer network element, wherein, the data packet carries the time stamp information, the data packet sequence number information and the cumulative data packet length information etc. For one or more continuous data packets which need the concatenation processing of the radio link control (RLC) protocol layer, the upper layer network element identifies the one or more data packets with the same time stamp information, and the one or more data packets identified with the same time stamp consist in a data burst, a data burst could be a group of continuous data packets or a single data packet;
S204, every lower layer network element receives the above MBMS service data packets, wherein, RLC concatenation processing is performed for the data packets in the same data burst rather than in different data bursts;
S206, the lower layer network elements start to process the data packets in the same data burst according to their sequence numbers at the time indicated by their indentified time stamps;
S208, the lower layer network elements check whether there is lost packet or not and the number of lost packets according to the data packet sequence number information in each data packet;
S210, the lower layer network elements detect the cumulative length of the lost packets according to the cumulative data packet length information carried in each data packet and construct virtual data packets.
Since the MBMS service data packets that the upper layer network element sends to each lower layer network element are the same, the lower layer network elements can perform the same processing to implement the synchronous transmission of the MBMS service among the cells in which all lower layer network elements locates.
With the above method, the lower layer network elements could construct virtual data packets according to the number and the total length of lost data packets when the lower layer network elements detect that there is lost data packet, wherein the number and the total length of the virtual data packets are the same as those of the lost data packets. After constructing the virtual data packets, the lower layer network elements add the constructed virtual data packets into the user plane protocol processing, and it acts as the lost data packets having not lost. However, the lower layer network elements do not send the user plane data blocks (RLC PDU or MAC PDU) including the virtual data packets to guarantee subsequent processing for the data packets in the same way as processing by the other lower layer network elements, and to avoid the radio interference among the adjacent cells due to the inconsistence between the virtual data packets and the actual ones.
It should be noted that the upper network element and one or more low layer network elements are identical or different in physical function, “upper layer network element” and “lower layer network elements” are only logic divisions and these network elements work together to implement the service synchronization function, that is, several identical or different physical network elements are divided into one upper network element and one or more low layer network elements according to their logic functions, and these network elements work together to send the MBMS service in cells where lower layer network elements are located in the way of multi-cell combination.
There are the following problems in the above-mentioned method for implementing the MBMS service synchronization among the cells where a plurality of network elements are located:
1. In the above-mentioned method for implementing the MBMS service synchronization among the cells where a plurality of network elements are located, each lower layer network element individually implements the RLC processing for the MBMS service data packets, which specifically comprises assigning RLC sequence number, RLC segmentation and concatenation. In normal conditions, each lower layer network element maintains the current RLC sequence number and performs the above RLC processing according to the currently received data packet. Since the initial RLC sequence number assigned by each lower layer network element could be synchronous through the configuration, the consistence of RLC sequence number assignment could be guaranteed by every lower layer network element in data processing. However, in the method for implementing the MBMS service synchronization among the cells where a plurality of network elements are located, when a lower layer network element restarts owing to management reasons or abnormal conditions, this lower layer network element cannot determine the right RLC sequence number, thus it cannot maintain RLC sequence number synchronization with other lower layer network elements which have not restarted.
2. In the process that the data are transmitted from the upper layer network element to the lower layer network elements, there might have lost data packets and these lost data packets could be continuous. According to the prior art, the lower layer network elements can detect the number and the total length of the lost data packets, and according to which construct virtual lost data packets, perform user plane processing for these virtual data packets, like that these data packets were not lost, thus to guarantee the consistence between the processing condition of the subsequently received data packets and that of the other lower layer network elements having no lost data packets, and also to guarantee its synchronization with other lower layer network elements after correctly receiving the subsequent data packets in the case that there are lost data packets.
However, the data packets of a previous data burst might not concatenate with the data packets in the data burst behind it in the same RLC PDU, if the data packets in the previous data burst do not fully occupy one RLC PDU or MAC SDU, the RLC PDU or MAC SDU is filled in the RLC or MAC layer, and the size of the filled data depends on the unoccupied space in the RLC PDU or the MAC PDU. In the condition that there are continuous lost data, the lower layer network elements cannot determine whether the several lost data packets belong to the same data burst or not, and also cannot determine whether these data packets should be concatenated with each other or be filled or not, that is, the size of the RLC PDU or MAC PDU space actually occupied by the lost data packets cannot be calculated correctly.
In the above condition, the lower layer network elements which have lost data or which have restarted cannot be RLC serial number synchronous with other lower layer network elements which operate properly, thus there is radio interference among the cells.